1. Field of the Invention
The present invention relates generally to improvements in diaphragm apparatus and more particularly to a diaphragm type pressure responsive switch or valve having hysteresis for preventing hunting.
2. Description of the Prior Art
A typical conventional pressure-responsive apparatus for use as a switch, valve or the like has a structure and operation as described hereafter with reference to FIG. 1. The pressure-responsive apparatus has a diaphragm 4 in a metal case 1. When pressure in a vacuum tank T rises above a predetermined level, that is, when a pressure difference between a first chamber 1b connected to the vacuum tank T and a second chamber 3b exposed to the atmosphere is smaller than a predetermined amount, the diaphragm 4 is biased to the right in FIG. 1 by a compressed spring 6. Then, a switch formed between case 1 and terminal 3 is ON. The spring 6 is held between a washer 7 on an inside wall and a spring receiving seat 51 fixed to the center of the diaphragm 4. A moving contact 5 is thereby caused to contact terminal 3 fixed to the center of a plastic cap 2. As a result, an electric circuit consisting of the motor M of a vacuum pump P and a power source 20 is closed thereby operating the vacuum pump. Thereafter, the pressure of the vacuum tank T reaches a predetermined low pressure, and the pressure difference between the first chamber 1b and the second chamber 3b becomes larger than a predetermined amount. The diaphragm 4 is then biased to the left in FIG. 1 as a result of the pressure difference now overcoming the force of the spring 6. The moving contact 5 separates from the terminal 3, that is, the switch opens, deenergizing the motor M of the vacuum pump P.
In such a conventional pressure-responsive apparatus, since the diaphragm 4 is driven by the pressure difference between the first chamber 1b and the second chamber 3b, a first pressure difference to make the moving contact 5 contact the terminal 3 and a second pressure difference to make the moving contact 5 separate from the terminal 3 are substantially the same. Thus, closing and opening of the motor circuit tend to occur too frequently, responding, in practice, to every slight change of pressure in the vacuum tank T. This induces hunting of the diaphragm 4 and moving contact 5, terminal 3 and motor M, tending to shorten the lifetimes of the moving contact 5, terminal 3 and motor M.